The invention relates in general to an apparatus for cutting out the vent of a fowl, having a hollow cylindrical knife concentrically rotatable about a center pin, which is adapted to be inserted into the fowl's vent opening in order to center the vent relative to the knife, and more in particular to an apparatus of this kind adapted to cooperate with an overhead conveyor with shackles for transporting the birds to be processed, which are hanging by the ankle joints from the conveyor shackles.
With any such apparatus, which serves to make a circular cut in the skin of the fowl around it's vent in order to sever the viscera from the skin, the problem always is that it must be avoided, that the viscera are touched by the knife and also that the vent, once it is cut out, rotates with the knife, which will twist and might rupture the gut attached to the vent. The regulations on hygiene in processing plants and in particular on prevention of cross contamination are so strict in many countries, that a bird of which the viscera are damaged is rejected as unfit for human consumption.
For this reason apparatusses were developed in which was tried to hold the vent against rotating with the knife and in many instances it was also tried to pull up the vent towards the knife, so that the knife would not have to be inserted into the fowl very deep, thereby lessening the danger of touching the underlying viscera with the knife.
In a number of known apparatusses vacuum is used to hold the vent, which however not only has the disadvantage that vacuum conduits easily get clogged, since apart from the vent also the faeces are sucked up, but is also undesirable from a hygienic point of view, especially since very often compressed air is used to clear clogged vacuum conduits, which could cause aerosol contamination.
In other known apparatusses and especially in automatic apparatusses cooperating with the overhead conveyor, mechanical means are used for pulling up and/or holding the vent. Although in this manner the use of vacuum is avoided, as a rule such mechanical means are very complicated, which leads to an elaborate and expensive construction and because of the great number of moving parts involved, such apparatusses again are susceptable to mechanical failures. Another disadvantage is that the known apparatusses make an oversized and irregular hole in the skin and the underlying layers of fat and meat, which not only causes yield losses but also make the subsequent steps of automatic processing more difficult and less efficient.
Although, when cutting out the vent, it would therefor be desirable to make the hole in the bird as small as possible, it is even more important to make sure that the so called "rose-bud", which is a gland connected with the gut and lying directly under the vent between the gut and the tail of the fowl, is cut out together with the vent. According to the official regulations the rose-bud, the scientific name of which is "Purse of Fabricius", must be cut out, since otherwise the shelf life of the processed poultry would be drastically reduced.
Obviously reducing the diameter of the knife would result in a smaller hole in the bird, but that would also mean that the bird and the knife must be very accurately positioned relative to each other when the center pin is inserted and during the cutting operation to ensure that the rose-bud and the vent are correctly removed.
Most of the known apparatusses have two cooperating positioning members, one of which grips the breast of the bird and pushes it under the cutting unit, whereas the other member acts as an abutment for the back of the bird, so that the vent opening comes to lie in the center line of the center pin of the knife. The first member may be a stationary guide bar or a pivotable member moving along with the overhead conveyor and pressing against the breast of the bird in it's upper or lower position, whereas the second member usually consists of a counter pressure plate moving along with the conveyor, which carries the fowl.
The use of a counter pressure plate like that has the disadvantage that the distance between the plate and the center line of the center pin and the knife must with great accuracy lie within very narrow limits. If this distance is chosen so small that the rose-bud will fall within the diameter of the knife with absolute certainty, the danger exists that the tail of the bird will hinder inserting the center pin into the vent opening or that the knife will cut into the spine of the bird when the vent is cut out. If on the other hand, to avoid this danger, the distance is increased the knife may miss the rose-bud so that it will not be cut out. This means that a compromise is necessary with the result that already a relatively small variation in the size of the birds will cause the choosen distance to be too small or too large.
In trying to solve this problem it has already been proposed to tilt the bird during the cutting operation by first positioning the bird so that the knife will enter the bird at an angle in the direction of the bird's spine in order to cut out the rose-bud, after which the bird is quickly tilted back before the knife reaches the spine, so that the knife penetrates the bird in parallel with the spine. Although this avoids that the tail gets in the way when the center pin of the knife is inserted, the danger of cutting into the spine remains or even increases, since in practice it is very difficult to pick the right moment for tilting the bird back and all the more so because this moment varies with the size of the birds. Apart from that additional means for tilting the bird make the apparatus complicated, vulnerable and expensive.